1. Technical Field of the Invention
The present invention relates generally to detecting flow transition and shear stress or skin friction of airfoils and more particularly to an apparatus and method for achieving active thermal isolation of such detection devices.
2. Discussion of the Related Art
The detection of flow transition between laminar and turbulent flow and of shear stress or skin friction of airfoils is important in basic research for validation of airfoil theory and design. These values are conventionally measured using hot film nickel sensors deposited on a substrate of polyimide such as "Kapton.RTM." brand polyimide produced by the E. I. Dupont de Nemours Company, which in turn is bonded to the surface of the aerodynamic body exposed to an airflow, e.g., an airfoil or wind tunnel model. The polyimide substrate provides sufficient electrical insulation to avoid shorts between the body surface and the resistance circuitry of the sensor. However, the polyimide substrate does not have sufficient thermal insulation characteristics. Substantial heat is accordingly lost via conductive heat transfer from the sensor to the intimately located body. This heat loss is of course worsened by the use of highly heat conductive materials to construct the aerodynamic body. This thermal loss affects both the sensor frequency response and resolution.
The thickness of the polyimide substrate may be increased in an attempt to provide better thermal insulation, i.e., passive thermal insulation. However, this increased thickness causes flow disturbances which contaminate the test environment.
Another proposal to thermally isolate the sensor from the model uses internal heat sources to heat the entire aerodynamic body surface. However, the thermal mass of the body is significantly larger than the sensor and the conductive heat transfer accordingly cannot be accurately controlled since the response time necessary for the body temperature change is significantly slower than the response time necessary for the hot film sensors. In addition, this proposal requires significant energy expenditures to heat the entire surface area as well as extensive modification of the interior region of the aerodynamic body, and especially that of a model, in order to house the heating system. Also, heating the entire test body may alter the basic flow state of the body under study.